Net energy, often called 'Energy Return On Investment' (EROI), is the energy an organism gains from food or sunlight, minus the energy it spends in collecting it. It works like 'net income', but in this case we are counting energy units - calories or kilowatt-hours - instead of money. Net energy is measured in a ratio, such as three-to-one (3:1), which would mean an organism spends one calorie of energy in order to get three calories of energy from its food. Every animal - including humans - needs surplus energy for its other activities: to keep its blood warm and run its body, to migrate or fix its shelter, to reproduce and look after its children, and so forth.

An organism must gain energy at the rate of more than 1:1 - if a trout in a stream, for example, used one calorie of energy in order to get only one calorie of energy back from its food, it would have zero net energy and would die. In a typical trout population, individuals who expend more energy than they gain die, and those with a positive net energy thrive. Wasting energy is a poor strategy in nature - to save energy, successful trout learn to tuck themselves behind rocks to avoid swift current and wait for their food to float by. Fish that migrate, such as salmon, must obtain food with at least a 3:1 net energy, burning no more than one calorie to obtain three calories in their food.

The goal of life is living in agreement with nature.

Zeno (335 - 264 BC)

Historical net energies

A Pleistocene hunter-gatherer community probably subsisted on a 4:1 or 5:1 net energy ratio, meaning that they burned up one calorie of energy for every four or five calories they gathered and consumed. We have no precise data, but researchers have been able to calculate this based on modern observations.

Richard Lee's study of the modern Kalahari !Kung people (The !Kung San: Men, Women and Work in a Foraging Society, Cambridge University Press, 1979) showed that highly successful, efficient foraging communities of 20-30 people could subsist at about 10:1 net energy. This relatively high net energy allowed cultures to explore their world and develop arts.

The first explosion in human population came not just because of agriculture, but with the domestication of animals. This allowed humans to exploit animals' net energy and use it for themselves. Humans also learned to exploit the net energy of other humans; the practice of slavery. If an ox or a slave can produce 5:1 net energy through its work, but the master keeps the ox or the slave barely alive - so that it effectively exists with a 1:1 net energy ratio - then it is the master who gets to keep the net energy left over. Kings, emperors and early industrialists became wealthy by extracting net energy from soil, resources, animals and other humans. Agriculture, animal husbandry, slavery and worker exploitation allowed ruling classes to acquire and hoard net energy, which they could then use to build castles and chariots, smelt steel, wage war, and stockpile more resources. The modern-day 'sweat shop' is a net energy harvesting scheme.

Small, modest pre-industrial farming could achieve 10:1 net energy, and in some cases could sustain this level of energy extraction from the soil by carefully rotating crops and returning nutrients to the soil. However, in nature, 'success' has its limits. As an organism - or a human society - becomes more 'efficient' and more effective at extracting net energy from its habitat, it tends to deplete the resources of a particular region. Historically, hunter-gatherers remained semi-nomadic to compensate. Many ancient settlements moved because they became too 'successful' and depleted their environment.

Canadian soil microbiologist Peter Salonius explains that humanity likely entered overshoot of the Earth's carrying capacity once it abandoned hunter-gathering lifestyles for crop cultivation, a trend which has reached a climax with industrial agriculture.

Oil, corn and history

The discovery of coal and oil again transformed humanity because it represented a new, unique net energy bonanza. The hydrocarbons buried in the Earth represent 500 million years of solar energy, captured by photosynthesis, converted to biomass, dropped to sea beds and forest floors and concentrated by gravitational pressure over millennia.

Today, each year, humanity burns about 5 million years' worth of this stored sunlight. This represents a huge, one-time withdrawal from the Earth's energy storehouse. This energy is so concentrated, however, that we experienced unprecedented 'net energy'.

The first wave of modern oil wells, from about 1920-1940, produced a net energy over 100:1. It cost one barrel of oil to produce 100 barrels. Suddenly those living in the industrial world had so much cheap net energy that they could literally dominate the entire world. World War II was, in part, fought over oil energy, particularly the Caspian, Middle Eastern and North African oil fields. Churchill acknowledged this when he said: "You have got to find the oil ... purchased regularly and cheaply in peace, and with absolute certainty in war."

In 1990, US vice president Dick Cheney said, regarding the Gulf War, "We're there because … that part of the world controls the world supply of oil, and whoever controls the supply of oil .. would have a stranglehold on … the world economy."

Energy is the capacity to do work. Net energy flow is a proxy for the real work done by any organism or community of organisms, including human society. Net energy is therefore a proxy for human economic activity.

However, today, oil no longer returns 100:1 net energy. We are now down to the 20:1, 15:1 and 10:1 oil fields, and we are digging into the 4:1 and 3:1 tar sands. This changes everything.

Soil, oil and trouble

The so-called 'green revolution' in 20th century agriculture was in fact a 'black revolution', driven with cheap oil. According to Dave Hughes, in Homer-Dixon's Carbon Shift, one barrel of oil is the energy equivalent of one human working nonstop for 8.6 years.

The challenge humanity faces, as Salonius documents in his study, is that our addiction to cheap energy has created a society that cannot maintain itself. Ultimately, we must still pull nutrients and energy from the soil, but soil requires highly complex ecosystems to properly recycle nutrients and avoid erosion.

As humans devised sophisticated methods of extracting food and energy from Earth's soils - the plow, the oxen, the slave, the diesel tractor - we deplete soils of nutrients and cause erosion. We currently lose about 6 million hectares of arable soil and create about 14 million hectares of new deserts each year. On top of this, we have created an agricultural system that depends on cheap, high-net-energy oil, which is disappearing.

Dr. David Pimentel at Cornell University has shown the net energy impact as we use more petroleum to grow food. In the 1700s, Pimentel shows, on a pre-industrial farm, corn could be grown at over 10:1 net energy, ten calories of corn from one calorie of farming effort. By 1910, because of the hydrocarbon cost, corn net energy was less than 6:1, and today it is less than 2:1. A trout does better than this in a stream, hiding behind its rock and picking off bugs.

But our net energy status is worse than that. Much food consumed in the industrialised countries now arrives with a negative net energy, meaning it takes more calories to grow, process and deliver the food than what it contains. Some industrialised meat and vegetables consumed in Europe and North America have reversed the pre-industrial 10:1 farms with a negative 1:10 net energy. In these cases it takes ten calories to deliver one calorie of food. In nature, this is a recipe for extinction.

Humanity has been able to get away with this temporarily because we burn 500 million years' worth of stored solar energy each year. None of this is sustainable. As humanity depletes the one-time storehouse of hydrocarbons, the net energy values drop like a stone - from the hydrocarbon heyday of 100:1, down to 10:1 and the 4:1 and 3:1 tar sands. The habits we learned in the cheap energy days will not survive in a society run on 4:1 tar sands oil.

The hydrocarbon age was a net energy bonanza for humanity that will not be repeated with the low net energy oil we find today, or by any 'alternative' fuel. A wind farm might achieve 18:1 net energy in excellent conditions, but could be as low as 4:1 or 5:1. Corn-based ethanol has a net energy barely above 1:1.

No species can sustain itself if it spends more energy in finding its food than benefitting from the energy the food actually contains. We must consume less energy and wean our agriculture off hydrocarbons if we hope to reach anything like real sustainability.

- RexWeyler

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